Box 3.

.

Transcranial magnetic stimulation

For TMS a brief strong current is delivered through a coil generating a magnetic field which traverses the scalp (Barker et al. 1985). In turn an electric field is produced that causes current flow leading to excitation within the neuronal tissue under the coil. The method is free of pain because the brain contains no pain receptors. Instead, for example, when applied over the visual cortex TMS pulses can elicit phosphenes, which are local light sensations (Barlow et al. 1947). TMS became a valuable tool in neuroscience as a single TMS pulse produces a focal suppression (“virtual lesion”) within the target area that can be used to infer its relevance for a given task (Walsh and Cowey 1998; Pascual-Leone et al. 2000). High-frequency pulses are often used to measure cortical excitability or to alter the cortical state when applied as pulse trains over longer time periods (Pascual-Leone et al. 1999; Di Lazzaro et al. 2004; Huber et al. 2013). The latter regime was used in our studies